public class Solution {
    public int numTrees(int n) {
        int[] dp = new int[n+1];
        dp[0] = 1;
        for(int i = 1; i <= n; i++){
            for(int j = 1; j <= i; j++){
                dp[i] += dp[j - 1] * dp[i - j];
            }
        }

        return dp[n];

    }
}
